Psyche is who your character is on the inside.  And it's the inside that counts.  It is what makes a character good or bad, defend the weak or abuse their power.  It is what makes them a unique hero despite the possibility of other characters with similar stats.  In short, the psyche is the character.   The psyche consists of spheres, beliefs, traits, and a character description.  Spheres are the subcultures or groups that a character is familar with.  Beliefs are the core statements of value, ideals, and motivations that drive the character.  Traits are adjectives describing personality quirks or other aspects of the character people might notice.  The Character Description is a brief paragraph describing the personality of the character, their history, and their background.  It should reflect and/or explain some or much of the beliefs and spheres, as well as help determine what stats are appropriate to pick.

         There are several tools that I have found helpful in 

determining or coming up with a psyche for a character or simply fleshing out one I have a basic idea for.  Arrete and Flaws gives some additional explanation of this idea, as well a list of motivations, gifts (called arrete- the greek word meaning excellences or virtues), and flaws.  One of the most helpful tools I have found in this process is the enneagram, as it provides a set of archtypes that have realistic motivations, fears, and areas of excellence, while also showing ways for that personality to develop into a better or worse person.